The electrical impedance of an element can be measured by connecting an alternating current signal of fixed frequency and amplitude to the element and measuring the current flowing through the element. However, AC current flowing through an element of fixed impedance will also change if the frequency, amplitude or harmonic content of the AC excitation signal changes.
One known method to ensure accurate impedance measurements is to utilize an AC signal source which is highly stable in frequency, amplitude and harmonic content. Stable sources of this type are often very expensive. Yet, it is only necessary for accurate impedance measurement that the total current flowing through the impedance be constant (i.e. unique) for a given impedance value. It is therefore possible to compensate for shifts in amplitude, frequency and harmonic content of the excitation signal by merely controlling the amplitude of the electrical test signal. Using this technique, the signal source need not be exceptionally stabilized in any one of (a) output frequency, (b) harmonic content or (c) amplitude because the output signal is automatically amplitude-corrected to compensate for any shifts in these parameters.
The prior art generally discloses such an amplitude-corrected alternating current signal source for providing test excitation to a variable impedance element. For example, Wescott et al (U.S. Pat. No. 3,533,286, issued Oct. 13, 1970) and Vogel (U.S. Pat. No. 3,901,079, issued Aug. 26, 1975) disclose tank quantity gauges using a capacitive sensor to sense liquid level in a tank. An AC oscillator is used to provide excitation to the capacitive sensor. A control signal is fed back to a control input of the oscillator to change the amplitude of the oscillator output voltage to compensate for changes in the dielectric constant of the liquid sensed.
Wescott et al (U.S. Pat. No. 3,580,074, issued May 25, 1971) discloses a similar capacitive sensor liquid level gauge in which a control signal dependent upon the temperature and dielectric constant of the liquid in the tank is applied to a control input of the oscillator.
Myers (U.S. Pat. No. 4,259,865, issued Apr. 7, 1981) discloses a fluid gauging system utilizing a capacitive sensing unit the capacitance of which varies according to liquid level. The sensing unit is connected to the sinusoidal output of an oscillator. The capacitance of the sensing unit is measured indirectly by measuring the current induced to flow through the sensing unit by the excitation signal.
A fixed capacitor in Myers is also connected to the oscillator and the current flowing through the fixed capacitor is sampled. Any changes in the current flowing through the fixed capacitor are attributed to changes in the excitation signal. The sampled current is first scaled and filtered and the resulting direct current voltage is then compared to a fixed reference voltage. The difference signal generated is used to control the amplitude of the oscillator output (in some way not shown) to compensate for the changes in the current flowing through the fixed capacitor. As a result, any changes in current flowing through the capacitive sensor are attributable solely to changes in the capacitance of the sensor. Although Myers also teaches a form of temperature compensation by placing the fixed reference capacitor in the same fuel tank environment as the variable level measuring capacitor, this is not in reality a practical alternative because such extra electrical elements are normally not permitted in the fuel tank (e.g. due to added air frame wiring requirements).
Prior art systems such as Myers thus have several disadvantages. For example, the Myers system requires two active elements just to generate the oscillator control signal. Perhaps more importantly, no practical compensation for changes in output signal resulting from temperature variations within the signal conditioning is provided. Furthermore, the overall oscillator and feedback control circuit of such prior art may not be readily realized in hybrid microcircuit form--depending upon the details of circuitry not shown. The present invention has none of these disadvantages.